Walking through the woods the other day I was surprised at just how many trees had holes in them. Some of them had birds flitting in and out of them and I could see plenty of insects around them.
The holes in trees are home to many species of birds including doves, starlings, owls, and nuthatches. Small mammals such as squirrels and mice also live there, as do amphibians and snakes. Insect larvae live in wet tree holes as do protozoans, crustaceans, and hexapods.
I found it really interesting and I am sure you will be amazed at the sheer variety of animals living in tree holes.
Tree holes can be formed by lightning strikes, fungi, or even by animals excavating them. These holes make fantastic, secure, hidden dwellings for many animals.
If you walk through woodlands anywhere in the United States or North America, you will most likely see trees with ringed holes, shallow depressions or large craters in them. These can all make a great home for many animals depending on whether the hole is dry or wet.
How Holes Are Formed
If the bark of a tree is damaged, then the wood underneath may be attacked by many types of fungi. Beetles can spread the fungi as they burrow into the wood.
The beetles and the fungi can cause the area to enlarge, forming one of the large holes you see in trees. These are one of the most common holes and are called rot holes.
The tree can be initially damaged by lightning, animal damage, or even bad weather such as high winds. Humans can also damage trees by pollarding or coppicing.
Depending on the position of the hole they can either stay dry or get extremely wet. When rain water fills them, the wood will start to rot, especially if there are leaves inside the holes.
The tree can callus around a hole and can sometimes close. The rotten cavity behind the hole stays however.
Some trees grow with shallow depressions on the surface. These are called pans are can form when tree crowding leads to distortion of branches and trunks. Although pans are shallow depressions they can hold water.
Both types of holes can be seen when walking through woodland, and can be seen especially on deciduous trees such as oak, ash and beech.
The Effect Of Water On Tree Holes
When rot holes or pans become filled with water they provide a habitat for many species of animal life. Protozoans, crustaceans, hexapods and microscopic rotifer can be found in almost all bodies of water, even holes in trees.
If the water is stagnant then this can attract the larvae of flies. There is plenty of food to support small wildlife as the rotting wood and leaves provide a great source of food supplying many species.
The habitats that water-laden tree holes supply can last for as long as the tree lives. If the hole dries out then the species living there may also die out or complete their life cycle quickly. However, this generally only happens in extremely parts of the continent as most can survive as long as there is some moist decaying material present.
What Lives In Wet Holes?
Many small species of animal that live in waterlogged holes can not be found elsewhere and grow in this habitat. By living in these holes in their immature stages they escape their natural enemies of beetles or dragonfly larvae.
Several species of mosquitoes live in wet holes and the holes they pick depend on their species. Some prefer clearer water whereas others prefer darker water rich in tannin.
Eggs are laid by the females into the water of the tree holes during summer. Eggs will either immediately hatch, although some do not hatch until the next spring. Once hatched they feed on material in the water, bacteria and algae. Mosquito larvae can swim but spend most of their time at the surface as they breathe air.
Like caterpillars into butterflies, mosquitos pass through larval stages and then into a pupal state. At this time they do not feed but remain active at the surface. Once the pupal stage has finished and the skin splits the adults emerge to mate.
The larvae of midges can also be found in the water in tree holes. Their life cycle is much like mosquitoes, entering four larval stages.
Woodlice, craneflies, snails and hoverflies may also live and breed in the water of tree holes.
What Lives In Dry Holes?
Dry holes offer a habitat for larger animals such as birds and bats, although some smaller anthropods also live there. Centipedes and spiders can be found in many holes throughout the year, although some species of insect use the holes as protection from the winter cold.
Many birds use dry tree holes to nest. Owls, doves, starlings, nuthatches and species of tits can often be found in dry holes.
Woodpeckers are known to make these holes and other species will use the holes they make in subsequent years. Other birds may line the holes with leaves, grass and other nesting material, although some may leave them unlined.
Larger species such as squirrels will also use dry holes to nest in, although they will generally use the holes higher up the trunk. Mice, stoats and small cats may use holes that are lower down the tree.
Many animals will use the holes in trees at some point to escape the cold or even predators.